The first time it comes up it defaults to turning on the Narrator, but there are options on that screen to select what you want to auto start so you can modify it to bring up the OSK automatically instead. With freeze panes activated scrolling with the scroll gesture on my mouse pad (two finger swipe) disconnects the content scroll position from the scroll bar position - I can see this happening when I scroll with the mouse pad the page scrolls but the scroll bar doesn't move. Another easy way to pull up the OSK is to use Windows and U key to bring up the Utility manager. This happens reliably if I have 'freeze panes' activated (e.g. In excel I get the issue of selecting a cell and it selects a different cell - this is seems to be where the scroll position of the content on the screen becomes disconnected from the scroll position of the scroll bar.

In Edge it appeared to be related to the zoom level if I reset my zoom level I could use the scroll bar otherwise the 'hit box' for the scroll bar and the visual position of the scroll bar were different

I've noticed this problem in Excel and Microsoft Edge.

No scroll bar in excel high os sierra